The principle of reducing unwanted disturbance by generating a disturbance with the opposite phase is well documented. The technique is often referred to as active control to distinguish from passive control where the elements of the system are incapable of generating disturbances. Nelson and Elliot, "Active Control of Sound", Academic Press (1992) review some of the work done to date.
The earliest technique in this field was done by P. Lueg who described an actuator and sensor coupled by a simple negative feedback loop in U.S. Pat. No. 2,043,416.
The main shortcoming of this system is that the disturbance can only be reduced over a limited range of low frequencies. This is because of the finite response time of the control system (the time taken for a signal sent to the actuator to cause a response at the sensor). The control loop cannot compensate for the phase shifts associated with this delay, and so only operates at low frequencies where the phase shifts are small. The gain of the feedback loop must be low at other frequencies to maintain the stability of the system. This is achieved by incorporating a low pass filter into the loop--which introduces additional delay.
The range of applicability of active control systems has been extended by the use of more modern adaptive control techniques such as those described by B. Widrow and S. D. Stearns in "Adaptive Signal Processing", Prentice Hall (1985). In U.S. Pat. No. 5,105,377, Ziegler achieves feedback system stability by use of a compensation filter but the digital filter must still try to compensate for the phase characteristics of the system. This is not possible in general, but when the disturbance has a limited frequency bandwidth the digital filter can be adapted to have approximately the right phase characteristic at the frequencies of interest. The filter characteristic therefore depends on the disturbance as well as the system to be controlled and must be changed as the noise changes.
One class of disturbances for which this approach can be successful is periodic disturbances. These are characterized by a fundamental period, a time over which the disturbance repeats itself. Disturbances are not often exactly periodic, but any disturbance where the period changes over a timescale longer than that over which the disturbance itself changes can be included in this class.
Several approaches have been put forth for controlling periodic disturbances including that described by C. Ross in U.S. Pat. No. 4,480,333. The patent describes a feedforward control system in which a tachometer signal is fed through an adaptive digital filter. There is no description of the form of the tachometer signal but it contains no information on the amplitude of the disturbance to be controlled and thus the filter must again be adapted in response to the disturbance. Chaplin et al, in U.S. Pat. No. 4,153,815, describe the method of wave form synthesis, where a model of one cycle of the desired control signal is stored and then sent repetitively to the actuator. Nelson and Elliot, infra, describe the equivalence of these two approaches in the special case where the period remains constant.
In U.S. Pat. No. 4,490,841, Chaplin et al recognize the benefit of splitting the stored waveform into its frequency components. The advantage of this step is that each frequency component can be adapted independently. This can improve the ability of the system to adapt to rapidly changing disturbances and can reduce the computational requirements associated with this adaption. Others have recognized this technique such as Swinbanks in U.S. Pat. No. 4,423,289 which describes the use of Frequency Sampling Filters and the equivalence of time or frequency domain weights.
In all of the above systems the filters have to be adjusted to cope with changing disturbances. This requires processing power and so adds costs to the control system. In addition, all of the systems above become increasingly complicated as the number of harmonics in the disturbance increase. This is a problem for disturbances which are impulsive in nature--such as the sound from the exhaust or inlet of an internal combustion engine.
Accordingly, it is an object of this invention to provide a control system for periodic disturbances that requires little or no adaption.
Another object of this invention is to provide a control system based in the time domain for canceling periodic disturbances.
A further object of this invention is to provide a unique system for controlling the cancellation of periodic disturbances wherein the amount of computation required does not increase with the number of harmonics to be controlled.